In certain types of industrial operations, such as warehousing, it is common practice to use a lift truck of the type known as an order-picker truck. Such trucks provide an extendible upright for a lift fork and an operator's station on the extendible upright. The operator controls the speed and steering of the truck and the lifting and lowering of the upright from this platform. It is desirable to provide safeguards against possible injury to the operator or damage to the vehicle in the event the operator attempts to operate or load the vehicle in an unsafe manner. In some of such vehicles, it is possible for the operator to manually control the elevation of the lift fork and loading in such a manner that the vehicle becomes unstable and may overturn in the event of excessive speed or turning the vehicle too sharply.
Heretofore, it has been proposed to provide a safeguard to prevent the operator from driving too fast with the upright in an extended or elevated position. The Thomas U.S. Pat. No. 3,486,333 describes a lift truck with a manually controlled extendible upright and an automatic system for preventing extension of the upright at high speed above a predetermined height. The automatic control system of this patent utilizes a limit switch located on the upright for disabling the high speed lift operation at a predetermined height. Above this height, only low speed lift operation is available. The Thomas et al U.S. Pat. No. 3,524,522 describes a lift truck with an extendible upright and means for limiting the speed of operation of the truck as a function of the height of the upright. In the control system of this patent, a speed control device of the solid state type is provided for varying the speed of the traction motor by varying the duration of power pulses or the frequency of power pulses from a battery to the motor. The current to the motor control device, which varies the duration or frequency of the power pulses, is controlled by first and second variable resistors in series with a normally closed limit switch, the vehicle battery and a control input of the speed control device. The first variable resistor is actuated by the manual accelerator lever and has a maximum resistance in the normal or low speed position. The second variable resistor is actuated by a float in the hydraulic reservoir of the lift system so that it has a maximum resistance with the extendible upright at its maximum height. Accordingly, the maximum speed at which the motor can be operated is no greater than the speed called for by the setting of either variable resistor, whichever is lower. More particularly, speed can be no greater than the value corresponding to the sum of the resistances of the two variable resistors. When the upright reaches a predetermined height, the normally closed limit switch which is positioned on the upright is opened and the motor is effectively deenergized to stop the truck.
A general object of this invention is to provide an improved automatic speed and lift control for safeguarding the operation of an industrial lift truck.